Dangerfield23

Can a vacuum leak cause the computer to hold the IAC valve open?

I recently installed a '99 Explorer intake, and after acceleration it idles at 2000 rpm. I can unplug the IAC and it idles fine. I plug it back in, and it jumps up to 2k. I installed a new IAC assembly, installed a new throttle body and TPS, installed a new computer, I have an adjustable bypass plate, but nothing has worked. I even tried a restrictor plate on the IAC, and it helped a little on the idle, but didn't fix it and caused some different problems so I took it off.

I was told that it could be a vacuum leak, but I figured a vac leak would idle high all the time. I can unplug the IAC and it idles fine. Is that characteristic of a vacuum leak?

When I installed the intake, the only thing I did different with the electronics is move the manifold air temperature sensor from the manifold to the air intake tube (between the MAF meter and the throttle body) because the explorer intake does not have a place for it.

4600cc

So:

1) With IDLE BYPASS off idle is normal.

2) With IDLE BYPASS on idle is high.


The should mean that idle bypass puts more air into the engine when it is on.


I want you to try this. Unplug hose from the rear that goes to the PCV valve. Put your finger both on the hose and the hole on the intake to plug them. Does this resolve high idle issue?


Also let's try this. On the side of the throttle body, there is a screw which adjusts how much open it can stay while throttle is out. Desconnect idle bypass so that it idles normally. Now adjust that screw until you hear engine chocking and possibly stalling. Connect idle bypass and crank. What did this do?




AL

Dangerfield23

Let me tell you what I've done with the PCV so I can drive this thing with a normal idle.

The first thing I tired was to install an orfice tube in the PCV vacume line to restrict the vacuum. It idled good at first, but after I gassed on it, it went back to idleing at 2k. It dosen't seem to make much difference if the PCV is plugged or not, the IAC still makes it idle high.

Then I completely disconnected the PCV and plugged the vacuum line, installed a filter in place of the PCV valve (like the old carbureted engines) and I unhooked the IAC. This is the only way I can run it with the IAC unhooked so it won't die when I put it in gear. However, I have to wait for it to warm up on a cold start before I can drive it.

As for the set screw on the throttle body, I can unhook the IAC and use the set screw to close the butterfly valve all the way and it idles a little low, but it will not kill it. Of course I do have an adjustable bypass plate on the IAC (this helps with erratic idle from the E303 cam) so that is probably why its not killing it with the butterfly valve closed.